This invention pertains to the production of superplastically formed, complex, metal alloy structures that exhibit improved bi-axial and transverse shear stiffness, and more particularly to the production of metallic sandwich structures superplastically formed from metal blanks and diffusion bonded to form the structure; and the procedure for producing such structures.
Superplasticity is the characteristic demonstrated by certain metals to develop unusually high tensile elongations with minimum necking when deformed within a limited temperature and strain rate range. This characteristic, peculiar to certain metal and metal alloys has been known in the art as applied to the production of complex shapes. It is further known that at these same superplastic forming temperatures the same materials can be diffusion bonded with the application of pressure at contacting surfaces.
The prior art relating to superplastic forming of metallic sandwich structures is exemplified by U.S. Pat. Nos. to Summers et al 3,924,793; and Hamilton et al, 3,927,817. Both of these patents disclose methods for superplastically forming metalic sandwich structures from metal blanks, joined at selected areas and expanded superplastically. Both of these disclosures are limited to performing all the bonding operations prior to forming. The inside worksheets are bonded to the outside face sheets and then the forming is accomplished by inducing tensile stress in the face sheets as by applying gas pressure between the face sheets and thereby drawing the interior sheets with the face sheets as they form outwardly. Since both of these patents teach diffusion bonding prior to forming, it is essential that some means be employed to prevent bonding in the areas to be formed. Summers et al controls bonding by the use of spacers at the point of bonding which act to separate the blank sheets and limit the areas to be joined. Hamilton et al employs maskants or stop-off material to those areas within the stack of metal blanks where no attachment or joining between the sheets is desired. The known maskants, of which Boron nitride and Yttria oxide are the most commonly used, present a serious impairment to the structure unless removed after forming as they produce embrittlement. In complex sandwich structures it is impossible to totally remove the maskant after forming as the surfaces are inaccessable. In addition to the disadvantage of contamination, the use of maskants is also limited to relatively simple concepts and relatively wide joint areas.
Hamilton et al further teaches continuous bonds with vent holes provided in masked areas to avoid unwanted differential pressures between compartments during forming which would distort the structure. During the forming process these vent holes themselves are superplastically stretched and enlarged resulting in a potential source for a fatigue crack or crack growth failure.
Probably the most serious singular limitation of the methods taught in the referenced patents is that the three sheet arrangement with stop-off for creating the sandwich structure is limited to producing what is known in the art as a truss core structure without transverse stiffening. The truss core members are limited to canted elements and the method cannot product vertical members.